Polycarbonate resins obtained by the interfacial polymerization of one or more aromatic dihydroxy compounds, e.g, bisphenol A, with phosgene in the presence of a proton acceptor are used as optical materials in applications such as various lenses, prisms, and optical-disk substrates because these resins are excellent not only in mechanical properties including impact resistance but in heat resistance and transparency.
However, the polycarbonate produced from bisphenol A as only the aromatic dihydroxy compound has the following drawbacks.
Since the polycarbonate has a high photoelastic coefficient and relatively poor melt flowability, it gives molded articles having enhanced birefringence and an Abbe's number as low as 30 although the refractive index thereof is high, i.e., 1.58. Namely, the performances of the polycarbonate are insufficient for wide use in applications such as photo-recording materials and optical lenses.
For the purpose of eliminating the above drawbacks of bisphenol A polycarbonate, the present inventors previously proposed an aromatic/aliphatic copolycarbonate resin (JP-A-10-120777; the term "JP-A" used herein means an unexaminied and published Japanese patent application). This aromatic/aliphatic copolycarbonate resin has excellent impact resistance and heat resistance, a low photoelastic coefficient, and a high Abbe's number and can hence be extensively used as an optical material. For producing such an aromatic/aliphatic copolycarbonate, the method known as a transesterification process is advantageously used because it is difficult to produce the polymer by the ordinary phosgene process. In the transesterification process, an aromatic dihydroxy compound, an aliphatic dihydroxy compound, and a carbonic diester, e.g., diphenyl carbonate, are subjected in a molten state to polycondensation through transesterification reactions.
In the transesterification reaction for polycarbonate production, polycondensation is usually conducted while heating the reaction mixture at a temperature of from 200 to 330.degree. C. The polymer being yielded is thus exposed to such a high temperature for a prolonged time period. Because of this heat history, the polymer yielded has an impaired color tone and other deteriorations. Namely, the prior art process has a drawback that a polymer of excellent quality is difficult to obtain. Consequently, the polycarbonate obtained by the transesterification process is unsuitable for use in fields where a satisfactory color tone is required.